A number of agents have been utilized as carrier molecules with limited success in imaging systems. In practice the carrier should be non-toxic and target site specific. Ideally there should be a mechanism for maintenance of the detectable compound or agent at the target site.
Radiopharmaceutical techniques currently used in non-invasive in vivo imaging methods are based upon the ability of the target organ to remove the radiopharmaceutical label from the circulation. These techniques utilize various substances to deliver radioactive compounds to desired target; such substances include substrates, substrate analogs, ligands, hormones, radionuclides, bifunction al chelate (linker groups containing a chelator at one end which is able to bind a heavy metal or radioisotope and a reactive group at the other end which can covalently attach to a target cell) and liposomes (Eckelman and Levanson, 1977, Intl. J. Appl. Radiat. Isot. 28: 67-82).
Other non-invasive techniques currently available are emission tomography, nuclear magnetic resonance imaging, and in vitro spectroscopy. A method employing isothiocyanate as a coupling agent has been used to attach fluorescent compounds to antibody molecules for use in fluorescence microscopy (Brandtzaeg, 1973, Scand. J. Immunol. 2: 273-290).